Assume that an actual yaw rate of a vehicle has a relationship of first order delay to a standard yaw rate of the vehicle and a coefficient multiplied to a vehicle speed in a time constant of the first order delay is referred to a time constant coefficient of steering response. A stability factor of the vehicle and a time constant coefficient of steering response t represent a turning characteristic of the vehicle. A stability factor of the vehicle and a steering-response-time constant coefficient can be estimated by using ARX (auto-regressive exogenous model) to estimate parameters a and b of a discrete-time transfer function from a standard yaw rate of the vehicle to an actual yaw rate of a vehicle.
In, for example, Japanese Patent Application Laid-Open (kokai) No. 2004-26074, a turning characteristic estimating device for a vehicle is described which estimates a standard yaw rate of a vehicle on the basis of running data when the vehicle turns; estimates parameters a and b of a discrete-time transfer function from a standard yaw rate of the vehicle to an actual yaw rate of the vehicle; estimates an estimation error ΔKh of stability factor of the vehicle on the basis of the parameters a, b and vehicle speed V; and sets the sum of an initial value of stability factor and the estimation error Δkh to an estimated value of stability factor of the vehicle.
The state values of a vehicle for calculating a standard yaw rate and an actual yaw rate are detected by sensors, the detected values of which can include detection error due to zero point offset of the sensors or the like. For that reason, in the conventional turning characteristic estimating devices such as that disclosed in the above-mentioned Laid-Open Publication, estimation of stability factor is liable to be affected by the detection error, which precludes enhancing estimation accuracy of stability factor.